Numerical Simulation Of Curing Deformation Of Thermosetting Resin Matrix Composites

Fiber reinforced resin matrix composites have varieties of applications in aerospace, automobile, architecture and sports equipment industries because of their notable performances, such as high specific strength and modulus, easy to design, good fatigue resistance and good corrosion resistance. The performances of this kind of composite are highly reliable on molding processes. Resin Transfer Molding (RTM) is one of the advanced manufacturing techniques and attracts more and more attention owing to its unique advantages. Aimed at the large aircraft development projects established by country mid-long-term scientific and technological development plan, research on the preparation technical of fiber composites by RTM technology to achieve high performance, low-defect, low-cost objectives is of great practical significance.The numerical simulation of the curing stage during RTM process is of significant importance in improving the product quality and reducing the cost. During curing process, because of the uneven distribution of the temperature field and cure degree field, the internal stress including thermal stress and chemical stress, can be induced, which can influence mechanical properties, shape and dimensional stability of composites, and may result in the composite plate warping, matrix cracking and matrix delimitation. Therefore, curing process attracts more and more attention and becomes one of the most important research priorities.During the curing process, the time-space distributions of the physical and mechanical properties and internal stress of composites show the characteristics of non-uniform, non-steady state and strong coupling, so we need to reveal their evolution laws, and develop the curing deformation control mechanism of the composites. Therefore, making the curing process of thermosetting resin as the study project, using polymer crosslinking reaction kinetic theory, polymer crosslinking reaction statistical theory, composite thermoelastic theory, composite constitutive theory, numerical heat transfer theory and finite element simulation method, the project attempts to carry out the integration research about the resin curing reaction-crosslinked structure-mechanical properties. On this basis, we aimed to explore the formation mechanism of curing shrinkage stress and thermal stress in composites, simulate the evolution of its internal stress and curing deformation and analyze the effects of the processing parameters and structural parameters on the curing deformation. In order to qualitatively and quantitatively explore the effects of processing parameters, structural parameters and material performance parameters on the internal stress and curing deformation, the local sensitivity analysis method is introduced and quantitatively analyze the evolutions and influencing degrees of processing parameters, structural parameters and material performance parameters during the curing process on the internal stress and curing deformation in the composites. On the basis of the local sensitivity analysis, the regulation mechanism of composite curing deformation during the composite preparation process can be proposed and realized.The main contents and conclusions were as follows:During the curing process, thermal stress is mainly controlled by the temperature field, and chemical stress is mainly controlled by the cure degree field. In order to understand the evolution of thermal stress and chemical stress during the curing process, accurately simulating the temperature and cure degree fields is an important procedure. According to coupling the heat transfer model and cure kinetic model, temperature and cure degree fields are simulated. The influencing factors of temperature and cure degree fields are analyzed, including cure temperature, convective heat transfer coefficient, thermal conductivity, heating rate, material thickness and fiber volume fraction.When simulating the temperature and cure degree fields, by analyzing the relationship among the temperature, time and degree of cure, we find that there is a linear relationship between the curing temperature and the logarithm of curing time under the iso-conversional conditions. By means of data fitting method, we propose a new simplified method of cure kinetic model. On the basis of this simplified method, the relatively simple relationships among time-temperature-cure degree are obtained, which simplify the original complex cure kinetic equation and can be easily used in engineering practice to avoid the cumbersome solution process.Time-Temperature-Transformation (TTT) diagram is a useful tool for analyzing and designing the curing process, and it has important research value. Most of the TTT diagrams are drawn through analyzing a large number of experimental data, which is time-consuming and laborious. There are also large systematic errors in experimental method. In this paper, we obtained the TTT diagram of a specific resin by numerical simulation method, which saves time greatly.On one hand, from the viewpoint of basic research, certain material structures could be designed to meet specific material performance requirements. Therefore, the research on the formation mechanism and evolution characteristics of material structures attracts more and more attention. On the other hand, from the viewpoint of industrial production, the appropriate processing conditions must be designed in order to obtain the specific material structures. In a word, it is important for both basic research and industrial production to understand the quantitative relationship among thermosetting resin curing processes, crosslink structures and mechanical properties. In this paper, using such disciplines as the polymer crosslinking reaction statistical theory, resin cure kinetic, heat transfer theory and so on, the basis equations of temperature field, cure degree field and crosslink structure field are established; the evolutions of temperature, degree of cure, crosslink structure and mechanical properties are analyzed; and the integrated research of resin curing processes, crosslink structures and mechanical properties are realized.We analyze the evolution of the thermal and physical properties of the composites during the curing process, and indicate the computing method and evolution of modulus, density and thermal expansion coefficient, specific heat and thermal conductivity. After that, aiming at a specific example, we analyze comparatively evolution of the elasticity modulus, shear modulus and shrinkage strain of resin and composites. Finally, we analyze the formation and development processes of the internal stress during the curing process.The processing parameters and structural parameters which influence the curing deformation are analyzed. These factors include cure temperature, cooling rate, convective heat transfer coefficient, the fiber volume fraction, lay-up styles, the curing reaction heat, curing shrinkage, viscoelastic effect of the resin and so on. All kinds of resin materials are more or less viscoelastic. Otherwise, in our paper, due to the computer resource constraints and saving the computing time, we choose the linear elastic model in most examples. While in the analysis of resin viscoelastic effect on curing deformation, we design several groups of comparison cases to analyze the difference between the linear elastic model and the viscoelastic model. We find out that the difference between the results which are calculated by the two models respectively is not significant when the fiber volume fraction is relatively high and the resin curing shrinkage is relatively low. That means the resin viscoelastic effect is not significant.After the analysis of the effects of processing parameters and structural parameters on internal stress and curing deformation, in order to explore the effects of processing parameters and structural parameters on curing deformation, the local sensitivity analysis method is introduced. The results indicate that fiber volume fraction, curing shrinkage, lay-up styles and cure temperature have significant influence on curing deformation. After the analysis of the influencing degree of several factors by the local sensitivity analysis method, we propose the advices of controlling the curing deformation focusing on the most important factors of curing deformation.